My ISP had some network issues today, and these manifested as a very strange problem. For some reason, the only websites accessible were those belonging to the ISP and to Google. However, all sites that I tried were resolving using DNS (I tried using both my ISP's and Google's DNS servers in conjunction with nslookup), they just would not load or respond to pings.
Every Google-owned website that I tried to access worked flawlessly, including Youtube and Google+. I could even access Google sites hosted outside my geographic area (India), such as google.de and google.co.za. It wasn't due to Google being cached by my ISP as Google+ continued to update in real-time, and I very much doubt that my ISP is caching all of Youtube.
I couldn't access any other sites hosted within India, or even within my city, so it doesn't look like a severed cable was to blame. Traceroute showed requests leaving my ISP's network as well.
Does anyone know of a network issue that could cause such a problem? My normal suspect in cases like these would be the nameservers, but I eliminated those as a problem.